docs: def sed(self, photon_energy, distance=1 * u.kpc):
          """Spectral energy distribution at a given distance from the source.
  
          Parameters
          ----------
          photon_energy : :class:`~astropy.units.Quantity` float or array
              Photon energy array.
  
          distance : :class:`~astropy.units.Quantity` float, optional
              Distance to the source. If set to 0, the intrinsic luminosity will
              be returned. Default is 1 kpc.
          """
          if distance != 0:
              out_unit = "erg/(cm2 s)"
          else:
              out_unit = "erg/s"
  
          photon_energy = _validate_ene(photon_energy)
  
          sed = (self.flux(photon_energy, distance) * photon_energy ** 2.0).to(
              out_unit
          )
  
          return sed